Step by step, subtract 3 from 8.
Step 1: Clear register A.
Step 2: Clear register B.
Step 3: Flip the 8 switch (8).
Step 4: Load into register A.
Step 5: Flip the 2 and 1 switches (3).
Step 6: Set Route to register B.
Step 7: Load into register B.
Step 8: Flip the Invert switch (Invert actually inverts register B's output and adds one to the result, utilizing the first bit's carry.)
Answer = 5!
The "real" way to perform subtraction is to wrap the last bit state around to the first bit, this is called end-round-carry. I didn't care to add the relays required to shift the results so you just ignore the last bit on this adder for the result.